Multilayered Cellular Metallic Glass Structures

ABSTRACT

Multi-layered cellular metallic glass structures and methods of preparing the same are provided. In one embodiment, the cellular metallic glass structure includes at least one patterned metallic glass sheet and at least one additional sheet. The at least one patterned metallic glass sheet may include multiple sheets connected together to form a group of sheets, and the structure may include a group of sheets sandwiched between two outer sheets. The patterned metallic glass sheets may be patterned by thermoplastically forming two- and/or three-dimensional patterns in the metallic glass sheets. The metallic glass cellular structures are useful in a wide variety of applications, including but not limited to blast protection applications, energy absorption applications, structural support applications, biomedical implant applications, heat exchanger applications, thermal management applications, electrical shielding applications, magnetic shielding applications, and debris and radiation shielding for aerospace and outer space applications.

CROSS REFERENCE TO RELATED APPLICATION

The current invention is a continuation-in-part of U.S. patent application Ser. No. 11/841,702, filed Aug. 20, 2007, the disclosure of which is incorporated herein by reference. The application also claims priority to U.S. Provisional Application No. 61/384,641, filed Sep. 20, 2011, the disclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

The invention is directed to multilayered cellular structures of metallic glass.

BACKGROUND OF THE INVENTION

Cellular crystalline metal structures and methods of their manufacture are known, and have been used in a variety of applications, including impact/blast amelioration systems, heat dissipation media, and acoustic isolation applications. These cellular crystalline metal structures typically include sheets of aluminum or other metal and include corrugated metal sheets sandwiched between and bonded to unformed sheets, thereby producing stiff, low density cellular metal structures. The ability of a cellular structure to absorb imparted kinetic energy is directly dependent on the solid material's specific strength, i.e., yield strength divided by density. Accordingly, cellular structures made of low specific strength solids, such as those used in the corrugated metal sheets, do not absorb large amounts of imparted kinetic energy. In addition, the materials used to produce the corrugated metal sheets in these structures exhibit limited solid-state formability, making the corrugated sheets difficult to produce.

SUMMARY OF THE INVENTION

The present invention is directed to multi-layered cellular structures of metallic glass, which can be used for a wide variety of applications, such as blast protection, impact mitigation, energy absorption, structural support, biomedical implants, heat exchangers, thermal management, electrical shielding, magnetic shielding, and debris and radiation shielding for aerospace and outer space applications. Metallic glasses exhibit densities comparable to conventional crystalline metals, but have far superior yield strengths. Consequently, metallic glasses offer unparalleled improvements in specific strength and energy absorption when used in cellular applications. Also, unlike crystalline metals, metallic glasses exhibit excellent formability when heated above the glass transition temperature and retain their mechanical properties when cooled to room temperature prior to crystallization. According to embodiments of the present invention, planar amorphous metals are processed into extremely low density, high strength, and plastically deformable cellular cores for cellular structure construction. Owing to the excellent micro-replication capabilities of metallic glasses, structures requiring close tolerances can be easily formed, such as protrusions to allow snap assembly. In addition, micro-formation of metallic glass ribbons can be performed to fabricate cellular structures having micro-cores exhibiting high toughness and ductility but resistant to elastic buckling.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present invention will be better understood by reference to the following detailed description when considered in conjunction with the attached drawings in which:

FIG. 1A is a schematic depicting a method of patterning a metallic glass sheet according to one embodiment of the present invention;

FIG. 1B is a schematic depicting a planar metallic glass sheet with slits according to one embodiment of the present invention;

FIG. 2A is a schematic depicting a method of stretching a metallic glass sheet according to one embodiment of the present invention;

FIG. 2B is a schematic depicting a two-dimensionally patterned metallic glass sheet according to one embodiment of the present invention;

FIG. 3A is a schematic depicting a method of corrugating a metallic glass sheet according to one embodiment of the present invention;

FIG. 3B is a schematic depicting a three dimensionally patterned metallic glass sheet according to one embodiment of the present invention;

FIG. 4A is a schematic depicting the assembly of metallic glass sheets according to one embodiment of the present invention;

FIG. 4B is a schematic depicting groups of connected three dimensionally patterned metallic glass sheets according to one embodiment of the present invention;

FIG. 5 is a photograph of a Co₆₉Fe₄Ni₁Mo₂B₁₂Si₁₂ ribbon, where the inlaid photographs depict plastic deformation of the ribbon produced by a knurling tool at room temperature;

FIG. 6 is a photograph of a Co₆₉Fe₄Ni₁Mo₂B₁₂Si₁₂ ribbon after thermoplastic corrugation according to one embodiment of the present invention, where the inlaid photograph depicts a cellular metallic glass structure including the corrugated ribbon;

FIG. 7A is a perspective view of a metallic glass sheet according to one embodiment of the present invention;

FIG. 7B is a perspective view of a metallic glass sheet according to yet another embodiment of the present invention;

FIG. 8 is a perspective view of a metallic glass cellular structure according to one embodiment of the present invention;

FIG. 9A is a schematic view of a metallic glass cellular structure according to another embodiment of the present invention;

FIG. 9B is a perspective view of a metallic glass cellular structure according to another embodiment of the present invention;

FIG. 10A is a perspective view of a metallic glass core according to one embodiment of the present invention;

FIG. 10B is a perspective view of a metallic glass core according to another embodiment of the present invention; and

FIGS. 11A to 11C are perspective views of metallic glass cellular structure according to yet another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to cellular metallic glass structures and to methods of preparing the structures. In one embodiment, a metallic glass structure includes at least first and second sheets, where at least one of the first and second sheets is a metallic glass sheet. According to one embodiment, for example, a structure includes at least one metallic glass sheet sandwiched between two outer sheets. In another embodiment, the at least one metallic glass sheet may be sandwiched between two outer sheets and the structure may further include at least one intermediate sheet. The use of at least one metallic glass sheet, for example, that is properly configures, e.g., corrugated or shaped, to form a fracture-resistant and buckling-resistant cellular core, provides a structure that is low in density, high in strength, and has good plastic deformability, all of which result in a high energy absorption capability. These properties are unequalled by similarly constructed structures of crystalline metal sheets.

A cellular structure, as would be recognized by one of ordinary skill in the art, is an assembly or network of solid plates or sheets that are arranged to form edges or faces of unit cells. (See, e.g., L. J. Gibson, and M. F. Ashby, Cellular Solids: Structure and Properties. 2^(nd) Ed. 1997: Cambridge University Press, Cambridge, UK, p. 1-2, or H. N. G. Wadley, “Multifunctional Periodic Cellular Metals,” Philosophical Transactions of the Royal Society A, Vol. 206, pp. 31-68 (2006), the disclosures of which are incorporated herein by reference.) The present invention is directed to cellular structures, which for the purposes of this invention are defined as assemblies of metallic glass sheets arranged to form a lattice that comprises a plurality of unit cells, each cell having faces that are wider than the thickness of the sheets, and as wide as the widths of the sheets. An exemplary cellular core (wherein cellular core is a unit cell of the cellular structure) is shown in FIG. 10, and exemplary lattice structures are shown in FIG. 11.

In one embodiment, the metallic glass sheet may be formed into a metallic glass core, which can be an in-pane loaded cellular core (FIG. 10A) or an out-of-plane loaded cellular core (FIG. 10B). The metallic glass core can be designed to maximize the energy absorption capability of the resulting structure by ensuring that the core fails by plastic yielding when dynamically loaded, rather than by brittle fracture or elastic buckling. For the core to evade brittle fracture, the thickness of the metallic glass sheet should be below the “plastic zone thickness” of the material. The “plastic zone thickness” is the thickness below which the material resists crack formation, and is proportional to the square of the fracture toughness over the yield strength of the material. More specifically, for the purposes of such an embodiment, the plastic zone radius (r_(p)) is defined as:

r _(p) =K _(lc) ²/πσ_(y) ²  (EQ. 1)

where, K_(lc), is the mode I fracture toughness of the metallic glass solid, and, σ_(y), is the plastic yield strength of the metallic glass solid. If the thickness of a metallic glass plate is larger than “plastic zone thickness” (r_(p)), the plate will fracture catastrophically if σ_(y) is reached upon bending or impact.

The plastic zone thickness for metallic glasses typically varies from few micrometers to several hundred micrometers on going from brittle to tough alloys. (“Plastic zone thickness” values for several metallic glasses are listed in M. F. Ashby and A. L. Greer, “Metallic Glasses as Structural Materials,” Scripta Materialia, Vol. 54, pp. 321-326 (2006), the entire content of which is incorporated herein by reference.) For example, amorphous Zr_(41.2)Ti_(13.8)Ni₁₀Cu_(12.5)Be_(22.5) exhibits r_(p)˜0.3 mm, corresponding to σ_(y)=1800 MPa and K_(lc)=55 MPa.m^(1/2). By comparison, 304 stainless steel exhibits σ_(y)=500 MPa and K_(lc)=150 MPa.m^(1/2), which result in r_(p)˜30 mm, that is, two orders of magnitude larger than that of the metallic glass. Correspondingly, a 1-mm thick plate made of 304 stainless steel will exhibit a very high resistance to fracture upon bending or impact. Therefore, despite their very, high strength, metallic glasses are at a considerable disadvantage in terms of fracture resistance compared to traditional materials such as steels. Nevertheless, if the cross sectional thicknesses of a part, e.g. a sheet, is smaller than r_(p), then fracture can be resisted. To ensure resistance of a metallic glass sheet to brittle fracture, the sheet thickness should not exceed the “plastic zone thickness” of the metallic glass. Therefore, the thickness of the sheet will depend on the metallic glass chosen for sheet formation.

Another structural parameter that can be considered is the geometry of the core itself. In one embodiment, for example, the core can be configured to evade elastic buckling by ensuring that the characteristic “core aspect ratio” (which can be approximated by the core height in the direction of loading divided by the sheet thickness) should preferably not exceed a critical value at which core buckling becomes possible. The critical “core aspect ratio” is determined by the elastic strain limit of the material as well as the core design geometry. The elastic strain limit of metallic limits is about 0.02, a value much larger than that of most crystalline metals. Therefore, due to higher elastic strain limits, a metallic glass core should generally have a lower aspect ratio than a crystalline metal core of the same design geometry in order to avoid elastic buckling.

The manner in which different core design geometries determine the criteria for core buckling is discussed in H. N. G. Wadley, N. A. Fleck, and A. G. Evans, “Fabrication and Structural Performance of Periodic Cellular Metal Sandwich Structures,” Composite Science and Technology, Vol. 63, pp. 2331-2343 (2003), the entire content of which is incorporated herein by reference. To ensure that the core can resist elastic buckling, the “core aspect ratio” should not exceed the critical value associated with core buckling. Therefore, the “core aspect ratio” of the sheet will depend on the chosen core design geometry and the direction of applied load. Accordingly, when sheets with small thicknesses are chosen, the cellular cores are preferably fabricated with small heights as well in order to maintain the “core aspect ratio” below the critical value associated with core buckling. One such case can arise when a brittle metallic glass characterized by a microscopic “plastic zone thickness” is chosen. In that case, microscopic sheet thicknesses are required to evade brittle fracture, and accordingly, microscopic core heights are required to evade elastic buckling. In such a case, micro-corrugation of micro-sheets can assist in fabricating fracture-resistant and buckling-resistant micro-cores.

Provided that the cellular structure geometry meets the aforementioned criteria for bypassing both fracture and buckling, then the structure would be expected to fail by plastic yielding. The failure stress under plastic collapse would be higher than the failure stresses that would have occurred had the structure failed by either buckling or fracture.

The metallic glass sheet of the structure may be made of any suitable metallic glass alloy. Non-limiting examples of suitable metallic glass alloys include Fe-based alloys, Co-based alloys, Mg-based alloys, Al-based alloys, Zr-based alloys, Au-based alloys, Pt-based alloys, Ni-based alloys, Cu-based, Ti-based, Pd-based alloys and rare earth-based alloys. In particular, one nonlimiting example of a suitable Fe-based alloy is Fe₈₀C₈P₁₂, one nonlimiting example of a suitable Co-based alloy is Co₆₉Fe₄Ni₁Mo₂B₁₂Si₁₂, one nonlimiting example of a suitable Mg-based alloy is Mg₆₅Gd₁₀Cu₂₅, one nonlimiting example of a suitable Al-based alloy is Al₈₅Ni₆Fe₃Gd₆, one nonlimiting example of a suitable Zr-based alloy is Zr₅₇Nb₅Cu₁₅Ni₁₃Al₁₀, one nonlimiting example of a suitable Au-based alloy is Au₄₉Ag_(5.5)Pd_(2.3)Cu_(26.9)Si_(16.3), one nonlimiting example of a suitable Pt-based alloy is Pt_(57.5)Ni_(5.3)Cu_(14.7)P_(22.5), one nonlimiting example of a suitable Ni-based alloy is Ni₆₀Nb₃₅Sn₅, one nonlimiting example of a suitable Pd-based alloy is Pd_(77.5)Cu₆Si_(7.5), and one nonlimiting example of a suitable rare earth-based alloy is La₅₅Al₂₅Ni₂₀.

In addition to the at least one corrugate metallic glass sheet, the structures according to embodiments of the present invention further include at least one additional sheet, such as an outer sheet or an intermediate sheet. The material of this additional sheet may be any suitable material. Non-limiting examples of suitable materials for this sheet include polymers, epoxies, glasses, wood, ceramics, metals (such as high strength sheet metals), metallic glasses (such as those described above) and composites thereof. In addition, in one embodiment, the additional sheet may be made of the same material as the metallic glass sheet. In embodiments including more than one additional sheet, the additional sheets may be made of the same material or may be made of different materials.

The at least one metallic glass sheet of the structure should preferably also have a thickness below the “plastic zone thickness” of the material, and is patterned to produce a 2D and/or 3D lattice configuration. As noted above, the sheet can be formed into a cellular core with a “core aspect ratio” below the critical value associated with core buckling. In one embodiment, for example, the metallic glass sheet 12 includes a 2D structure such as slits or small openings 13 in the sheet, as shown in FIG. 7A. Although shown as slits or openings cut into the metallic glass sheet, the 2D pattern may be any suitable pattern. For example, as shown in FIG. 5, as an alternative to slits or openings in the sheet, the 2D pattern may be an imprint in the surface of the sheet. FIG. 5 depicts a metallic glass sheet subjected to plastic deformation at room temperature from a knurling tool.

In another exemplary embodiment, the metallic glass sheet 12 includes a 3D pattern, such as a corrugated pattern 15, as shown in FIG. 6, Although the 3D pattern is illustrated as a generally sinusoidal pattern, any suitable 3D pattern may be used. For example, in addition to a generally sinusoidal shape, the 3D pattern may be a zigzag or similar shape.

In yet another embodiment, the metallic glass sheet 12 includes both a 2D and a 3D structure. For example, a sheet may include both slits or openings 13 and a corrugated pattern 15, as shown in FIG. 7B.

The at least one additional sheet may also be patterned to produce a 2D and/or a 3D structure in the sheet. The 2D and 3D patterns discussed above for the metallic glass sheet are also useful for the at least one additional sheet.

According to one embodiment of the present invention, as shown in FIG. 8, a metallic glass structure 10 includes at least one patterned metallic glass sheet 12 sandwiched between two outer sheets 14. In one embodiment, as shown in FIG. 9A, the at least one metallic glass sheet includes at least two metallic glass sheets 12 stacked on top of each other or otherwise combined to form a group of metallic glass sheets 12 a. The group of metallic glass sheets 12 a may be sandwiched between two outer sheets 14 to complete a metallic glass structure 10 a, as shown in FIG. 9A, Alternatively, groups of metallic glass sheets 12 a may be separated by one or more intermediate sheets 16 to form a multi-tiered metallic glass structure 10 b, as shown in FIG. 9B.

In another embodiment of the present invention, a method of making a metallic glass structure includes first making a patterned metallic glass sheet by heating a planar metallic glass sheet to a temperature conducive to thermoplastic forming. This temperature may be any temperature between the glass transition temperature (T_(g)) and the crystallization temperature (T_(x)) of the material making up the planar metallic glass sheet. Then, appropriate pressure is applied in order to produce the desired 2D and/or 3D pattern in the sheet. In one embodiment, the pressure may range from about 1 to about 10,000 MPa. After forming a pattern, the metallic glass sheet is cooled prior to crystallization.

FIGS. 1A, 2A and 3A depict exemplary methods of preparing two-dimensionally and three-dimensionally patterned metallic glass sheets according to an embodiment of the present invention. As shown in FIG. 1A, a 2D pattern is first formed in a planar metallic glass sheet 120 a as shown in FIG. 1B by creating slits 130 in the sheet with a cutting tool 170, such as a laser, to form a two-dimensionally patterned metallic glass sheet 120 a. The two-dimensionally patterned metallic glass sheet 120 a is then trimmed to size with a suitable tool, such as a laser. Then, as shown in FIG. 2A, the two-dimensionally patterned sheet 120 a is heated to a temperature conducive to thermoplastic forming and stretched or otherwise expanded to a desired length. Depending on the 2D pattern applied to the sheet, the stretching or expansion may create a system of open cells, a system of closed cells, or an imprinted surface having a surface texture dependent on the 2D pattern. The two-dimensionally patterned metallic glass sheet 120 a as shown in FIG. 2B is then heated to a temperature conducive to thermoplastic forming and three-dimensionally patterned to form a three-dimensionally patterned metallic glass sheet 120 b, as shown in FIG. 3B.

The metallic glass sheet may be three-dimensionally patterned by any suitable means. Nonlimiting examples of suitable such means include blow molding, injection molding, investment molding, superplastic forming, hot forging, explosive impact forming, stretching, bending and folding.

In one exemplary embodiment, as shown in FIG. 3A, the metallic glass sheet is three dimensionally patterned by passing it between two roller drums 200 a and 200 b. The roller drums 200 a and 200 b are generally cylindrical and have teeth 210 a and 210 b, respectively, on their lengths and circumferences. The teeth 210 a of the first roller drum 200 a engage the teeth 210 b of the second roller drum 200 b. The two-dimensionally patterned metallic glass sheet 120 a is passed between the teeth 210 a of the first roller drum 200 a and the teeth 210 b of the second roller drum 200 b. Upon passing between the teeth 210 a and 210 b of the roller drums 200 a and 200 b, the two-dimensionally patterned metallic glass sheet 120 a is three-dimensionally patterned by pressure exerted on the sheet 120 a by the teeth 210 a and 210 b of the roller drums 200 a and 200 b. Although the above method is described and illustrated in connection with a sheet that is both two-dimensionally and three-dimensionally patterned, it is understood that the method may be modified to create sheets that are either only two-dimensionally or only three-dimensionally patterned. For example, to make a sheet that is only two-dimensionally patterned, the second step of heating the sheet and the step of passing the sheet through the roller drums may be omitted. Also, to make a sheet that is only three-dimensionally patterned, the steps of cutting slits in the sheet and expanding the sheet may be omitted.

The teeth 210 a and 210 b of the roller drums 200 a and 200 b may take any suitable shape. For example, as shown in FIG. 3A, the teeth may be generally trapezoidal in shape to form a sheet having a three dimensional pattern including generally trapezoidally shaped peaks and valleys. Alternatively, the teeth may be generally circular or semicircular, generally ovular or semi-ovular, generally square, generally rectangular, or generally triangular in shape. However, the teeth are not limited to these shapes and may take any shape suitable to create the desired 3D pattern in the sheet.

In one embodiment of the present invention, as shown in FIG. 3B, the three-dimensionally patterned metallic glass sheet includes small protrusions 220 acting as either snap fit fasteners or locator registration pins. When viewed from one side of the metallic glass sheet, the protrusions 220 are indeed protrusions extending outwardly from the surface of the sheet. However, when viewed from the opposite side of the sheet, the protrusions 220 are indentations in the surface of the sheet, These protrusions 220 enable fast and secure attachment of the metallic glass sheets to one another to form groups of metallic glass sheets for use in the construction of metallic glass structures. To form these protrusions 220 in the metallic glass sheet, the roller drum 200 a includes protrusions 230 a in the spaces between the teeth 210 a, and the teeth 210 a include protrusions 230 b. Also, the roller drum 200 b includes indentations 240 a in the spaces between the teeth 210 b, and the teeth 210 b include protrusions 240 b. Upon rolling the roller drums 200 a and 200 b, the protrusions 230 a on the roller drum 200 a engage the indentations 240 b on the teeth 210 b of the roller drum 200 b, and the protrusions 230 b on the teeth 210 a engage the indentations 240 a on the roller drum 200 b. When the metallic sheet is passed through the roller drums 200 a and 200 b, the protrusions 230 a and 230 b force a portion of the metallic glass sheet into the indentations 240 b and 240 a, thereby forming the small protrusions 220 in the peaks and valleys of the metallic glass sheet.

Metallic glass sheets having 2D and/or 3D patterns may be formed thermoplastically in the supercooled liquid state, as shown in FIG. 6, which depicts a thermoplastically corrugated Co₆₉Fe₄Ni₁Mo₂B₁₂Si₁₂ ribbon. Such ribbons can be used to make cellular structures (shown in the inlay in FIG. 6) having a cellular density of about 0.17 g/cc, which corresponds to a porosity of 98%.

Metallic glass structures may be assembled in any suitable manner. As noted above, the structures may include one or more patterned metallic glass sheets sandwiched between two outer sheets, or may include groups of patterned metallic glass sheets separated by intermediate layers and sandwiched between two outer sheets. In embodiments including more than one patterned metallic glass sheet, the sheets may be stacked or otherwise connected or combined by any suitable means. Nonlimiting examples of suitable such means include welding, spot welding, laser welding, electron beam welding, soldering, use of adhesives, use of snap fits, diffusion bonding, transient melting, thermoplastic bonding in the supercooled liquid region, and bonding through an intermediate low glass-transition temperature alloy.

In embodiments in which the metallic glass sheets include snap fit protrusions (shown in FIG. 3B), the metallic glass sheets can be connected through the snap fit protrusions 220. FIG. 4A depicts one method of connecting the metallic glass sheets including the protrusions 220. As shown, each metallic glass sheet includes protrusions 220 a on the peaks and protrusions 220 b on the valleys. When connecting two sheets, the protrusions 220 a on the peaks of a top sheet are aligned with the protrusions 220 b on the valleys of a bottom sheet. The protrusions 220 b are snap fitted into the indentations of the protrusions 220 a to connect to top and bottom sheets. Any number of metallic glass sheets may be connected in this manner to form groups of metallic glass sheets, such as the one shown in FIG. 4B.

After connecting the desired number of patterned metallic glass sheets, the resulting groups of patterned metallic glass sheets may be sandwiched between two outer sheets, or may be separated by intermediate sheets to form multi-tiered structures that may then be sandwiched between two outer sheets. The patterned metallic glass sheets may be connected to the outer sheets or intermediate sheets by any suitable means. Nonlimiting examples of suitable such means include welding, spot welding, laser welding, electron beam welding, soldering, use of adhesives, use of snap fits, diffusion bonding, transient melting, thermoplastic bonding in the supercooled liquid region, and bonding through an intermediate low glass-transition temperature alloy.

To make metallic glass structures using patterned metallic glass sheets including snap fit protrusions, the top-most metallic glass sheet in a group of metallic glass sheets may omit the protrusions 220 a on the peaks. The sheets still include the protrusions 220 b in the valleys to facilitate attachment to the lower sheet, but the protrusions 220 a on the peaks may be omitted to improve connection of the top-most sheet to the outer sheet of the structure. To fabricate a top-most patterned metallic glass sheet with protrusions 220 b on the valleys but excluding protrusions on the peaks, the roller drums 200 a and 200 b may be modified accordingly. For example, the roller drum 200 a includes protrusions 230 a in the spaces between the teeth 210 a, but the teeth 210 a do not include any protrusions. Also, the roller drum 200 b does not include indentations in the spaces between the teeth 210 b, but the teeth 210 b include protrusions 240 b. Upon rolling the roller drums 200 a and 200 b, the protrusions 230 a on the roller drum 200 a engage the indentations 240 b on the teeth 210 b of the roller drum 200 b. When the metallic sheet is passed through the roller drums 200 a and 200 b, the protrusions 230 a force a portion of the metallic glass sheet into the indentations 240 b, thereby forming the small protrusions 220 only in the valleys of the metallic glass sheet.

The multi-layered cellular metallic glass structures according to the present invention may be used in any suitable application. Non-limiting, examples of suitable such applications include consumer electronic casings and frames, blast protection applications, energy absorption applications, structural support applications, biomedical implant applications, heat exchanger applications, thermal management applications, electrical shielding applications, magnetic shielding applications, and debris and radiation shielding for aerospace and outer space applications.

EXEMPLARY EMBODIMENTS

In one exemplary embodiment, the multi-layered cellular metallic glass structures are directed to frames and casings for consumer electronic devices. Owing to their very high strength and low modulus, metallic glasses have a very high scratch resistance and elasticity, along with a good corrosion resistance, and as a result are thought to be attractive materials for frames of consumer electronics devices. For example, amorphous Zr_(41.2)Ti_(13.8)Ni₁₀Cu_(12.5)Be_(22.5) exhibits a yield strength of 1800 MPa, which is nearly 4 times that of 304 stainless steel. Such high strength is in fact believed to be more than adequate for electronics frames applications. Despite these attractive properties, however, metallic glasses generally exhibit low fracture toughness with extreme sensitivity to flaws, and consequently, metallic glass frames are often prone to brittle fracture upon bending or impact.

Specifically, if the thickness of a metallic glass plate is larger than r_(p), the plate will fracture catastrophically if σ_(y) is reached upon bending or impact. r_(p) for metallic glasses typically varies from few micrometers to several hundred micrometers on going from brittle to tough alloys. For example, amorphous Zr_(41.2)Ti_(13.8)Ni₁₀Cu_(12.5)Be_(22.5) exhibits r_(p)˜0.3 mm, corresponding to σ_(y)=1800 MPa and K_(lc)=55 MPa.m^(1/2). By comparison, 304 stainless steel exhibits σ_(y)=500 MPa and K_(lc)=150 MPa.m^(1/2) which result in r_(p)˜30 mm, that is, two orders of magnitude larger than that of the metallic glass. Correspondingly, a 1-mm thick plate made of 304 stainless steel will exhibit a very high resistance to fracture upon bending or impact. Therefore, despite their very high strength, metallic glasses are at a considerable disadvantage in terms of fracture resistance compared to traditional materials such as steels. This shortcoming could potentially be overcome by fabricating plates with thicknesses smaller than r_(p) such that they can resist fracture. Fabricating ultra-thin metallic glass plates however has been met with considerable processing challenges, as the pressure requirements for molding a viscous metallic glass liquid in a long thin channel can be extremely high.

In the consumer electronics casing embodiment, cellular architectures, such as those presented in FIG. 11A to 11C, (1000) (See H. N. G. Wadley, “Multifunctional Periodic Cellular Metals,” Philosophical Transactions of the Royal Society A, Vol. 206, pp. 31-68 (2006), the disclosure of which is incorporated herein by reference), are proposed to be fabricated at least partially from metallic glass and used as frames for consumer electronics devices, provided that the structures are designed to provide adequate toughness without significantly compromising strength. Specifically, as described above, the structures consist of one or more metallic glass sheets (1010) patterned and/or arranged in a lattice (1012) configuration between two flat sheets (1014). In this embodiment, at least the lattice layer is formed of a metallic glass, and, of particular importance, the metallic glass sheet is designed with thicknesses smaller than the “plastic zone thickness” (r_(p)) of the metallic glass, as determined by EQ. 1, and the lattice is designed to exhibit a plastic collapse strength that is at least 50% that of the metallic glass solid.

In an exemplary embodiment, consider a honeycomb panel with a hexagonal lattice (shown schematically in FIG. 11A) made of amorphous Zr_(41.2)Ti_(13.8)Ni₁₀Cu_(12.5)Be_(22.5) (σ_(y)=1800 MPa). The panel has height h=0.6 mm and the struts have thickness t=0.15 mm. Since the thickness of the metallic glass throughout the structure is below r_(p), the structure can be expected to adequately resist fracture upon impact or bending and thus fail by plastic yielding.

Under such plastic yielding failure, the plastic collapse stress σ of an exemplary cellular structure with a hexagonal honeycomb lattice loaded out-of-plane can be calculated as follows (See, e.g., L. J. Gibson, and M. F., Cellular Solids: Structure and Properties. 2^(nd) Ed. 1997: Cambridge University Press, Cambridge, UK, pp. 155-157, the disclosure of which is incorporated herein by reference.):

σ=5.6σ_(y)(t/l)^(5/3)  (EQ. 2)

where σ_(y) is the plastic yield strength of the material, t is the thickness of the sheet and l is the width of the cell face. Substituting t=0.15 mm, l=0.6 mm, and σ_(y)=1800 MPa, one obtains σ=1000 MPa. Therefore, the structure will exhibit a plastic collapse strength that is more than 50% of the metallic glass plastic yield strength (and twice the strength of 304 stainless steel), while being able to resist fracture by bending or impact.

Interestingly, a cellular panel will also have much lower density than a. monolithic plate of the same thickness, which is highly desirable in consumer electronics applications. The ratio of the density of the structure ρ to the density of the monolithic solid ρ_(s) for an exemplary cellular structure with a hexagonal honeycomb lattice can be calculated as follows (see L. J. Gibson, and M. F. Ashby, Cellular Solids: Structure and Properties. 2^(nd) Ed. 1997: Cambridge University Press, Cambridge, UK, p. 42., the disclosure of which is incorporated herein by reference.):

ρ/ρ_(s)≈2/√3)(t/l)  (EQ. 3)

Substituting t=0.15 mm, l=0.6 mm, one obtains ρ/ρ_(s) 0.3, that is, a cellular panel of such architecture will be 70% less dense than a monolithic plate of the same thickness.

As the collapse strength of the exemplary lattice is 56% of the metallic glass yield strength but the density is only 30% of the metallic glass density, the specific strength of the lattice, wherein specific strength is defined as strength over density, would be 85% greater than the metallic glass specific strength.

The plastic collapse strength (σ) of any lattice structure can be more generally expressed as σ=Aσ_(y)(t/l)^(n), where σ_(y) is the plastic yield strength of the metallic glass, t is the thickness of the sheet, l is the width of the cell face, and where A is between 4 and 7, more preferable about 6, and n is between 1 and 3, and more preferably about 2.

Also, the density (ρ) of any lattice can be more generally expressed as ρ=Bρ_(s)(t/l), where ρ_(s) is the density of the metallic glass, t is the thickness of the sheet, l is the width of the cell face, and B is between 1 and 3, and preferably about 2.

Moreover, as described above, fabricating a hexagonal metallic glass honeycomb panel with t=0.15 mm, h=0.6 mm will be considerably easier than fabricating a monolithic metallic glass plate with t=0.15 mm. This is because the panel can be fabricated by patterning a 0.6 mm metallic glass plate, as opposed to molding a long 0.15-mm metallic glass plate. Specifically, a metallic glass plate can be heated above the glass transition temperature into the supercooled liquid region and pressed against a mold lattice of standing hexagons such as to replicate (or micro-replicate) the mold lattice. Methods of heating the metallic glass plate include, but are not limited to, furnace heating, inductive heating (e.g. using an RF coil), or ohmic heating (e.g. using a capacitor as in US Patent Application 2009/0236017, the disclosure of which is incorporated herein by reference). Materials for the mold lattice include, but are not limited to, copper, bronze, aluminum, steel, silicon, machinable ceramic, sapphire.

Examples of possible lattice architectures are shown in FIGS. 11A to 11C. Although several embodiments are shown, it should be understood that any suitable cellular structure may be used with the current invention, including, for example, honeycomb, prismatic, trusses, textiles, and foam structures.

In turn, these cellular structures may be incorporated in to any suitable electronic frame or casing, including, for example:

-   A casing or frame comprising a body having walls defining at least     one enclosure, wherein the enclosure is designed to at least     partially encase at least one electronic component; -   A frame which further comprises at least one opening; -   A frame with an opening that is designed to allow access to the at     least one electronic component; -   A frame that comprises at least two individual pieces; -   A frame whose individual pieces are either fixedly or moveable     attached; -   A frame whose individual pieces are joined together by one of the     connector from the group consisting of: adhesives, screws, and     snap-connectors; -   A frame having at least one portion that is further coated with a     high-hardened material selected from the group consisting of: TiN,     SiC and diamond; -   A frame having at least one portion that is anodized; -   A frame having at least one portion that is anodized to provide     rainbow colors; -   A frame in the form of a case for a device selected from the group     of: cellular phone, PDA, portable computer, and digital camera; and -   A frame that provides at least partial electronic interference     protection to the electronic component.

While the present invention has been illustrated and described with reference to certain exemplary embodiments, those of ordinary skill in the art understand that various modifications and changes may be made to the described embodiments without departing from the spirit and scope of the present invention as defined by the following claims. 

1.-29. (canceled)
 30. A cellular structure comprising at least one first metallic glass sheet; and at least one second sheet disposed on the at least one first metallic glass sheet such that the at least one first metallic glass sheet and the at least one second sheet form a lattice comprising a plurality of three dimensional cells, each cell having an aspect ratio defined by a height in a direction of loading divided by a thickness of the first metallic glass sheet, the aspect ratio adapted to evade buckling of the cellular structure.
 31. The cellular structure according to claim 1, wherein the aspect ratio is determined by an elastic limit of the metallic glass and the geometry of the cell.
 32. The cellular structure according to claim 2, wherein the elastic limit of the metallic glass is about 0.02.
 33. The cellular structure according to claim 1, wherein the at least second sheet comprises at least two sheets that are at least partially flat, wherein the at least one first metallic glass is disposed between the at least two sheets.
 34. The cellular structure according to claim 1, wherein the at least first sheet has a thickness of less than the plastic zone radius of the metallic glass.
 35. The cellular structure according to claim 1, wherein the geometry of the lattice is configured such that the plastic collapse strength of the lattice is at least 50% of the plastic yield strength of the metallic glass.
 36. The cellular structure according to claim 1, wherein the geometry of the lattice is configured such that the density of the lattice is less than 50% of the density of the metallic glass.
 37. The cellular structure according to claim 1, wherein the geometry of the lattice is configured such that the specific strength of the lattice is greater than the specific strength of the metallic glass.
 38. The cellular structure according to claim 1, wherein the plastic zone radius (r_(p)) of the metallic glass is determined by the equation: r_(p)=K_(lc) ²/πσ_(y) ², where K_(lc) is the mode 1 fracture toughness of the metallic glass, and σ_(y) is the plastic yield strength of the metallic glass.
 39. The cellular structure according to claim 1, wherein the plastic collapse strength (σ) of the lattice is determined by the equation: σ=Aσ_(y)(t/l)^(n), where σ_(y) is the plastic yield strength of the metallic glass, t is the thickness of the sheet, l is the width of the cell face, and where A is between 4 and 7 and n is between 1 and
 3. 40. The cellular structure according to claim 9, wherein A is about 6 and n is about
 2. 41. The cellular structure according to claim 1, wherein the density (ρ) of the lattice is determined by the equation: ρ=Bρ_(s)(t/l), where ρ_(s) is the density of the metallic glass, t is the thickness of the sheet, l is the width of the cell face, and B is between 1 and
 3. 42. The cellular structure according to claim 11, wherein B is about
 2. 43. The cellular structure according to claim 1, wherein the at least one metallic glass lattice comprises a material selected from the group consisting of Fe-based alloys, Co-based alloys, Mg-based alloys, Al-based alloys, Zr-based alloys, Ti-based alloys, Au-based alloys, Pt-based alloys, Ni-based alloys, Pd-based alloys and rare earth-based alloys.
 44. The cellular structure according to claim 1, wherein the at least second sheet that is at least partially flat comprises a material selected from the group consisting of polymers, epoxies, glasses, wood, ceramics, metals, metallic glasses and composites thereof.
 45. The cellular structure according to claim 1, wherein the at least second sheet that is at least partially flat comprises the same metallic glass as the lattice.
 46. The cellular structure according to claim 1, wherein the lattice has geometry selected froth the group consisting of honeycomb, prismatic, hexagonal, square, triangular, diamond, and nuvtrusses. 